A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In order to create, for example, a device on the substrate, it is often necessary to apply a plurality of patterns to a target portion of the substrate. The plurality of patterns may be overlaid on top of one another. In order for the resultant device to work correctly, the patterns must be overlaid on top of one another with a certain degree of accuracy. This is known as an overlay requirement. Any errors in the positioning of overlaid patterns are known as overlay errors.
Patterns are overlaid on top of one another using alignment marks located on or in one or more patterns or layers previously applied to the substrate. The accuracy with which patterns are overlaid is therefore dependent upon the accurate positioning of alignment marks, and also the accurate determination of the positions of those alignment marks when using the alignment mark as a reference (or guide) for the application and positioning of subsequently applied patterns. If the alignment marks are not located on a surface which is level with respect to the substrate surface, but is instead oriented at an angle with respect to the substrate surface, the determination of the positions of the alignment marks may be adversely effected. This may lead to an increase in the overlay error of subsequently applied patterns. Similarly, changes in the thickness of layers covering an alignment mark may also lead to an inaccurate determination of the position of the alignment mark, and therefore an increase in the overlay error. The error will be proportional to an angle at which a radiation beam is incident on the layer, and which is used to determine the position of the alignment mark.
It is desirable to provide, for example, a lithographic apparatus and method that obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.